As well known to those skilled in the art, wastewater treatments are divided into a percolating filter method, an activated sludge method, a contact oxidation method, etc. In the percolating filter method, wastewater after first sedimentation flows on crushed stones provided with a mucous membrane of microorganisms or other contact medium, thereby causing organic matters in the wastewater to contact a biofilm to purify the wastewater.
Here, the mucous membrane of the microorganisms mainly includes bacteria, and in a favorable condition, includes higher organisms such as sludge worms, larvae of flies, rotifers, etc. Nitrifying bacteria may exist at the deep portion of the mucous membrane to carry out nitrification.
The percolating filter method generally belongs to aerobic treatments. However, the mucous membrane of the contact medium used in the percolating filter method includes aerobic bacteria within the outer layer of 0.1 mm to 0.2 mm and anaerobic bacteria in the inside. The percolating filter method has an excellent capacity for coping with a variation in a wastewater load to be treated, does not requires the return of sludge, only generates a small amount of sludge, allows equipment to be constructed and managed at reduced cost and to be easily operated. However, in case that the contact medium has a large depth, the percolating filter method has drawbacks of generating offensive odor and a clogging in the contact medium, thereby causing flies swarming therein.
The activated sludge method is substituted for the above percolating filter method. In the activated sludge method, organic matters dissolved in wastewater are taken and decomposed by organisms in an aeration tank, etc., and the organisms grown thereby are solidified and precipitated in a final precipitation tank. A portion of the precipitated organisms is returned as activated sludge to the aeration tank, and the remaining portion of the precipitated organisms is discharged as waste sludge.
Since the activated sludge method has excellent processing efficiency in wastewater treatment, this method has been widely utilized now. However, in case that the activated sludge method is used to treat wastewater on a small-scale, this method cannot cope with the variation in a wastewater load to be treated, requires expert skill in adjusting the amount of sludge (microorganisms) and supplied air and excessive power cost, and generates sludge bulking.
The contact oxidation method may be referred to as a fixed-type activated sludge method, and is one of methods for treating organic wastewater using a biofilm. In this method, an aeration tank is filled with a contact medium, and the biofilm contacts the wastewater and the surface of the contact medium so that organic matters in the wastewater are removed.
Since the amount of organisms generated in the contact medium in the contact oxidation method is determined by a wastewater load condition, it is difficult to adjust the amount of the organisms. Further, since the sludge is created in proportion to the amount of the load, in case that the load is large, the contact oxidation method has problems that the load condition is limited such as the clogging of the contact medium, and it is difficult to uniformly aerate the aeration tank, thus generating a dead space therein.
Many researches on the aforementioned wastewater purification have been carried out. Korean Utility Model Reg. Appln. No. 1993-2216 discloses an equipment and method for purifying wastewater using anaerobic/aerobic contact circulation methods. The wastewater purification equipment disclosed by this application serves to purify wastewater using microorganisms, and comprises a precipitation-separation tank for precipitating solid substances and impurities contained in influent wastewater and then separating the precipitated substances from the wastewater, first and second anaerobic filtering tanks filled with an anaerobic filter medium for improving the removal of nitrogen components and organic matters from the wastewater, first and second contact aeration tanks filled with a contact medium including an aerobic microorganism layer and provided with an aerator for separating the organic matters from the wastewater, and a center wall into which the wastewater from the above tanks is introduced.
The above wastewater purification equipment further comprises a precipitation tank provided with an air lift pump for recycling the decant water and a measuring device, a disinfection tank for finally disinfecting the wastewater, and an effluence tank for discharging the finally-treated wastewater. The precipitation tank, the disinfection tank, and the effluence tank are connected from an influx side to an efflux side of the equipment.
However, since the above wastewater purification equipment is not provided with a device and process for periodically back-washing the anaerobic filter medium and discharging the back washing water, when the equipment is used for a long period of time, the anaerobic filter medium is clogged and a contact dimension between the wastewater and the anaerobic filter medium is reduced. Thus, the purification efficiency of the anaerobic filter medium is reduced. Further, solid substances are accumulated onto the anaerobic filter medium for a long term, thus reducing the processing capacity of the tank and inducing offensive odor. Moreover, recycled water is supplied from the precipitation tank to the anaerobic filtering tank, thereby allowing sludge (sedimentation) to be accumulated in the anaerobic filtering tank and degrading the anaerobic condition of the anaerobic filtering tank. When the water flows out of the precipitation tank, pollutants remaining in the water are also discharged to the outside.
In order to solve the above problems, Korean Patent Appln. No. 2000-54525 discloses a wastewater treatment apparatus using the anaerobic/aerobic contact circulation method and an attached filter method. As shown in FIG. 1, wastewater is recycled through a circulation tank and a contact aeration tank and treated under in anaerobic and aerobic conditions, finally treated by an attached filter tank, and subsequently flown out. Accordingly, it is possible to effectively discharge sedimentation, which degrades the treatment efficiency, to supply microorganisms in an activated state to the wastewater in the tanks, and to maximize the treatment efficiency due to the removal of residual pollutants.
The wastewater treatment apparatus disclosed by the Korean Patent Appln. No. 2000-54525 comprises precipitation-separation tank 91; first and second circulation tanks 92 and 93, each provided with an anaerobic filter medium for decomposing water overflowing the precipitation-separation tank 91 using anaerobic microorganisms and a back-washing pipe for removing sludge excessively attached to the filter medium; first and second contact aeration tanks 94 and 95 located adjacent to each other, each provided with an aerobic filter medium for decomposing water overflowing the second circulation tank 93 using aerobic microorganisms, a back-washing pipe for removing sludge excessively attached to the aerobic filter medium, and a diffusing pipe; a precipitation tank 96 for returning a part of the water treated by the second contact aeration tank 95 to the first circulation tank 92 via a circulation pipe, precipitating sludge from the water overflowing the second contact serration tank 95, and transferring the sludge via an excessive sludge transfer pipe using an air lift pump to the precipitation-separation tank 91 to discharge the sludge; an attached filter tank 97 for treating residual pollutants from the water overflowing the precipitation tank 96 using an attached filter medium; a back-washing water transfer pump for separating sludge excessively attached to the attached filter medium via a back-washing pipe installed within the attached filter tank 97 and simultaneously transferring the separated sludge to the precipitation-separation tank 91 via the back-washing transfer pipe to discharge the sludge; and a blower 90 for performing the aeration, the back-washing, and the air lift functions required by each tank.
In the above wastewater treatment apparatus, wastewater is influent into the precipitation-separation tank 91 so that solid substances and impurities contained in the wastewater are precipitated in the precipitation-separation tank 91. The wastewater first-treated by the precipitation-separation tank 91 overflows the precipitation-separation tank 91 and is fed to the first and second circulation tanks 92 and 93 so that nitrogen components are removed from the wastewater using anaerobic bacteria of the anaerobic filter media. Then, the wastewater, from which the nitrogen components are removed, overflows the second circulation tank 93 and is fed to the first and second contact aeration tanks 94 and 95 so that organic matters of the water are decomposed by aerobic bacteria. The wastewater, in which the organic matters are decomposed, is introduced into the precipitation tank 96 so that residual organic matters are precipitated in the precipitation tank 96. The wastewater overflows the precipitation tank 96 and is introduced into the attached filter tank 97 provided with the attached medium so that residual pollutants are removed by aerobic bacteria activated by supplied oxygen. Then, the wastewater treated through the above tanks is discharged to the outside.
This wastewater treatment apparatus has a complicated structure and purifies wastewater via multiple stages, thus requiring a large space for the installation. Further, such a wastewater treatment apparatus requires the supply of oxygen, thereby causing economical burden and troublesomeness to users. Particularly, the wastewater treatment apparatus does not completely purify the wastewater so that the purified water may be reusable, but only purifies the wastewater so that the purified water reaches an allowable level to be discharged, thus not having reusable effect of the resulted water.
Moreover, the above wastewater treatment apparatus determines its purification process to be finished when the wastewater passes through each tank just one time, and consequently discharges the processed water to the outside. Accordingly, the above wastewater treatment apparatus discharges the water from which pigments and odor are not completely removed, thus inducing water pollution and destructing an ecosystem.